1. Field of Invention
The present invention relates to a laminate electrical insulation part for an electrical device having a component, such as a core, provided with a plurality of slots to receive coils of current-carrying wire. Such laminate electrical insulation parts include, for example, a slot liner, a closure, a wedge, or a stick comprising a dielectric polymer film and at least one nonwoven sheet attached to each side of the dielectric polymer film solely by thermal bonding of the nonwoven sheets to the film.
2. Description of the Related Art
The Japanese Patent Publication Hei 11[1999]-170467 of Miki, et al. discloses a polyester composite material that can be used as a highly heat resistant electric insulating material characterized by the fact that it was prepared by thermally bonding on one surface or both surfaces of a polyester film a polyester nonwoven fabric made of polyester principal fibers and thermally fusible binder fibers having a melt point 5-50 degrees C. lower than the melting point of the principal fibers. The ratio of principal fibers to binder fibers in the nonwoven fabric should be in the range of 70:30 to 40:60, preferably 65:35 to 45:55. Therefore about half of the fibers at the surface of the nonwoven are binder fibers available for attaching the nonwoven sheet to the film. This publication further states that when the proportion of the binder fibers increases above 60 weight percent the heat resistance of the polyester composite material becomes lower. Therefore, adding additional binder fiber to the nonwoven sheet to increase the adhesion of the nonwoven sheet to the film is not a realistic option.
The integrity of the laminate structure is an important property for electrical laminate insulation parts. It is important that these laminate parts have adequate stiffness or rigidity, have a low surface friction coefficient, and that they do not readily delaminate, such as when slot liners are inserted into the slots of electrical device components or when conductors or wires come in contact with the slot liners during installation. Therefore it is desirable that both sides of the film be fully and uniformly bonded (i.e. over the film's entire surface) to the adjacent nonwoven sheets. Further, because stiffness or rigidity is important to these parts, generally highly crystallized films are used, for example bi-axially oriented PET film. This stiffness also makes it more difficult to adhere the film to the nonwoven sheets.
Because of these problems and requirements, most laminate electrical insulation parts use solvent-based thermoset adhesives to adhere nonwoven sheets to the polymer film. However, the commercial use of additional solvent-based adhesives generally requires continuous solvent-based processes and all the technological and safety complications related to dealing with volatile organics (ventilation, recuperation of the solvent, etc.). If the adhesive is solvent-free, similar complications are usually related to safety aspects of monomers, for they can also have a low vapor pressure, be volatile, and/or be a health hazard. Also, there is a perception by some in the industry that such adhesive laminates have a short shelf life; that is, they should be stored for only a few months prior to being installed in electrical devices. In addition there is a perception by some that parts made from adhesively-attached laminates may gradually and progressively delaminate while in service if the parts were improperly flexed, for example during installation, or if the adhesive bond was not properly achieved when the laminate was made. Still others perceive that the addition of an adhesive provides yet another material to the laminate part that because of differing thermal expansion coefficients is more apt to have delamination issues as the part cycles through periods of higher and lower temperature.
Therefore, what is needed is a laminate electrical insulation part comprising nonwoven sheets attached to a film wherein the nonwoven sheets provide uniform adhesion over the full interface between the nonwoven sheets and the film and other desirable attributes while avoiding the use of adhesives, solvents, or other environmentally unfriendly substances.